Matthew 26:69-70: Now Peter sat outside in the courtyard. And a servant girl came to him, saying, “You also were with Jesus of Galilee.” But he denied it before them all, saying, “I do not know what you are saying.”

Luke 22:56-57: And a certain servant girl, seeing him as he sat by the fire, looked intently at him and said, “This man was also with Him.” But he denied Him, saying, “Woman, I do not know Him.”

Matthew says that Peter's first denial was to multiple people, but Luke says that Peter's first denial was to one person.